Dialkoxymagnesiums, especially diethoxymagnesium, are used as solid catalyst components for polymerization of olefins such as propylene. Spherical or ellipsoidal diethoxymagnesium is used, and although the mean particle size, represented by D50, is known in patent documents to be 10-100 μm, in practice it is no greater than 50 μm. This is because increasing the mean particle size lowers the catalyst strength and results in problems such as micronization during use.
It is known that using an olefin polymerization catalyst obtained using diethoxymagnesium granules yields an olefin polymer with a shape that is enlarged and analogous to the catalyst shape. Therefore, a spherical catalyst shape also produces a spherical polymer. The obtained polymer preferably has a spherical shape for a superior flow property, but the polymerization catalyst must then have a spherical shape as well, and therefore it is important for the shape of the diethoxymagnesium as the catalyst component to also be spherical.
The particle shapes of olefin polymers obtained using polymerization catalysts employing diethoxymagnesium particles with particle sizes of about 50 μm are enlarged and analogous to the shape of the catalyst, but such polymers are in the form of fine powder which are still too small for direct molding with molding machines such as injection molding machines. The polymer fine powder obtained by such polymerization is therefore pelletized before being molded into a product. If the olefin polymer obtained by polymerization could be used for direct molding without passing through a pelletizing stage (step), to obtain a usable product, significant value in terms of cost would be achieved. It is therefore necessary to obtain diethoxymagnesium as spheres or ellipsoids with particle sizes of 80 μm or greater and preferably 100 μm or greater, and with a narrow particle size distribution range.
Methods for obtaining spherical dialkoxymagnesium by direct reaction between metallic magnesium and an alcohol have long been known and are described in, for example, Japanese Examined Patent Publication HEI No. 7-20898. In recent years, improved methods have been proposed wherein spherical dialkoxymagnesium obtained in the manner described above is contacted with an alkoxytitanium compound in an inert organic solvent to form a suspension and the solvent is then removed (Japanese Unexamined Patent Publication No. 2004-210683).
However, it is difficult to obtain spherical dialkoxymagnesium of a large enough size to eliminate the need for pelletizing of olefin polymers, and even when a large size is obtained the strength is extremely low. When an olefin polymerization catalyst is prepared using dialkoxymagnesium with such insufficient strength as the starting material, the starting material disintegrates during the preparation process, making it impossible to obtain a large-sized catalyst with uniform shapes. The polymer obtained using such a catalyst with non-uniform shapes will also have non-uniform particle sizes and a larger proportion of fine powder, while a larger proportion of the polymer will be non-spherical and the flow property will be impaired.
Also, the diethoxymagnesium production processes proposed in the prior art yield products whose content of fine particles with sizes of less than 1-10 μm is 5-8 mass % or greater of the total product. Because such particles form fine catalysts even after catalyst preparation, fine polymer powder is also abundantly present in the polymers obtained from them, and it interferes with the flow property. Consequently, diethoxymagnesium with a mean particle size of no greater than 50 μm and containing particles with fine particle sizes of no greater than 10 μm are used at the current time, even though it is desired to increase the particle size and minimize the presence of fine particles.